6m does sound quite long, although I’ve never heard of anyone trying. It’s great that you are doing your best to make provision for the hogs. I imagine the 13cm gap would be better, if possible, but also make as many hog holes as you can, so the hogs can find a way through the neighbouring area as well – especially as during building works they may not be able to get through even if you did leave a gap.
Also, make sure any holes for foundations, etc. have some sort of a way out at night in case a hog falls in by mistake. But also check the holes fairly frequently. I recall, years ago hearing about a family of hogs who were found in a fence post hole which had been left for a couple of days, who ended up with hypothermia.
Good luck with the extension. Let us know how you and the hogs get on.